1. Field of the Invention
The present invention relates to an acousto-optic element, a light deflector, a light beam scanning apparatus and an image recording apparatus. More specifically, the present invention relates to an acousto-optic element such as an acousto-optic modulator (AOM) and an acousto-optic deflector (AOD) using an acousto-optic effect, a light deflector using an acousto-optic effect, a light beam scanning apparatus for use in an image record and an image read by the use of this acousto-optic element and this light deflector, and an image recording apparatus for recording the image by the use of this acousto-optic element and this light deflector.
2. Description of the Related Art
Heretofore, an acousto-optic deflector (referred to as AOD below) for performing a light deflection by an acousto-optic element has been known. This AOD is arranged on an optical path of a light beam of, for example, a light beam scanning apparatus or the like. For example, the light beam to be scanned by the scanning apparatus is previously deflected by the AOD, so that the AOD is used for an application such as a correction of a light beam irradiation position to an irradiated object.
On the other hand, in the above-described AOD, the AOD is provided in which an anisotropic Bragg diffraction generated between an ultrasonic wave propagating in an anisotropic crystal and a light wave propagating in the anisotropic crystal is used so as to perform the light deflection. In such an AOD, a single crystal composed of a tellurium dioxide (TeO.sub.2) , which is capable of a low-power drive, is used as an acousto-optic medium. A transversal ultrasonic wave, which propagates in a 110! axial direction of a TeO.sub.2 crystal and displaces in a 1' 10! axial direction (where 1' denotes a direction of -1), causes the incident light wave in the TeO.sub.2 crystal to be diffracted. The AOD using the TeO.sub.2 crystal can obtain a higher acousto-optic performance index and a higher diffraction efficiency than the AOD using a lithium niobate or a rock crystal as the acousto-optic medium.
However, the On- 110! type light deflector dose not have flat change characteristics of the diffraction efficiency to a change in a frequency of the ultrasonic wave. Near a center frequency, the diffraction efficiency is considerably reduced, thereby resulting in a considerable reduction of light volume of a diffractive light. Thus, there has been a problem that an available frequency band is narrow. Furthermore, in order to obtain the high diffraction efficiency in the On- 110! type light deflector, the incident light on the light deflector is required to be a circular polarized light. In order to convert a straight polarized laser light emitted from a laser light source into the circular polarized light, a .lambda./4 plate is necessary. Therefore, there has been another problem that a cost is increased.
As the AOD which solves the above problems, an Off- 110! type light deflector in which a direction of propagation of the ultrasonic wave is largely tilted from the 110! axial direction of the TeO.sub.2 crystal (see Japanese Patent Application Laid-open No. 51-99039). According to the Off- 110! type light deflector, the reduction of the light volume near the center frequency that is the problem of the above On-110! type light deflector is solved. In addition, since the incident laser light may be the straight polarized light, the .lambda./4 plate is unnecessary.
However, in the Off- 110! type light deflector, since the direction of propagation of the ultrasonic wave is largely tilted from the 110! axial direction, the size of the TeO.sub.2 crystal used as the acousto-optic medium is required to be increased. Thus, the cost is increased.
On the other hand, in the light beam scanning apparatus, for the two-dimensional correction (movement) of the light beam irradiation position to the irradiated object or the like, two AODs are combined with each other. The two AODs are arranged so that the AODs may be in series along the optical path of the light beam and the deflecting directions of the light beam of the two AODs are perpendicular to each other (tandem arrangement). However, the problem such as an increase of cost as described above is more significant.